Field of the Invention
This invention relates to a method for manufacturing a semiconductor device. More particularly, the invention relates to a method for manufacturing a MOS (Metal Oxide Semiconductor) device with multiple threshold voltages in the semiconductor device.
Description of Related Art
The conventional transistor has only two kinds of status, which are "ON" and "OFF". Therefore, the conventional transistor provided in a memory device can only be used for storing a data of a bit, which is either "0" or "1". If there are N bits for storing, there must be an equivalent number of independent transistors to store these N bits data. The capacity for storing data per unit of volume in the memory device can not be efficiently increased as the semiconductor technique has an unexpected development.
For the above-described need, a MOS device with multiple voltages has been developed recently for using in the memory device. A conventional method for manufacturing these kinds of memory devices is forming a gate oxide layer with different thickness for a transistor in the memory device. The other conventional method is forming doped regions with different concentrations for a transistor in the memory device. These two conventional methods for forming a MOS device with multiple threshold voltages has the same feature, which extra masks are necessary for the multiple threshold voltages. The number of masks used in the manufacturing process is an index for the difficulty of the process. If more masks are necessary for a manufacturing process, it is difficult to control the reliability of the products and the reproduction of the pattern of the mask in the process. A choke point for manufacturing the MOS device with multiple threshold voltages is faced in the conventional methods.